Generally, a contrast detecting system is used as an auto focus (hereinafter, referred to as AF) system in TV cameras and video cameras. In the AF of the contrast detecting system, for example, a video signal is obtained by photographing an object, and a signal of a higher frequency component is extracted from the video signal by using a filter (electrical filter), so that a contrast degree (sharpness) of the image of the object is evaluated based on the signal of the higher frequency component. Then, a focus (focusing lens) position of a photographing lens is controlled to make the evaluated value (which is herein called “contrast value”) the largest or maximum (peak).
When a relationship between a focus position and a contrast value is illustrated in a graph (contrast value graph) with the focus position being along a horizontal axis and the contrast value being along a vertical axis, in photographing a certain object, a graph curve (hereinafter, referred to as contrast value curve) exhibits a unimodal distribution which has the peak of a contrast value at a focusing position for focusing. Thus, as a method for setting a focus at the peak point, a so-called hill climbing technique is known in which a direction of an increasing contrast value is detected so as to move a focus in the direction, and the focus is stopped at a position where no increase of the contrast value is detected. For example, Japanese Patent Application Laid-Open Nos. 63-074273 and 03-297282 describe an AF in a contrast detecting system.
A steepness of a unimodal distribution of a contrast value curve changes depending on conditions of an object or an optical setting of a photographing lens. For example, for a photograph of an object with a high contrast due to many high contrast patterns or contour lines, the unimodal distribution of a contrast value curve steeply slopes, and to the contrary, for photographing of an object with a low contrast due to less patterns or contour lines, the unimodal distribution of a contrast value curve gently slopes.
In addition, the steepness of a unimodal distribution of a contrast value curve changes depending on an aperture value in the case of a photographing lens of a rear focus type, and depending on a focal length and an aperture value in the case of a zoomable photographing lens which has an adjustable focal length of a front focus type. For example, in the latter case, when a zoom adjustment is set toward the tele side (a longer focal length) and an aperture is moved toward an opening side (a smaller aperture value), the unimodal distribution of a contrast value curve steeply slopes, while when a zoom adjustment is set toward the wide side and an aperture is moved toward an closing side (a larger aperture value), the unimodal distribution of a contrast value curve gently slopes.
When a focus position (focusing lens) is moved to a peak of a contrast value by using the above described hill climbing technique in AF, generally, the more gently the unimodal distribution of a contrast value curve slopes, the higher a focusing speed is, which is the movement speed of a focus (especially, the focusing speed in AF is called AF speed), and also the larger a position error between the position where the focus is stopped for the peak of the contrast value and the true focus position is, which results in a low focusing accuracy (which is called AF accuracy).
While, the more steeply the unimodal distribution of a contrast value curve slopes, to the contrary to the above case, the higher an AF accuracy is, but a higher AF speed makes it difficult to stop the focus at the peak of a contrast value, which easily causes various phenomena such as hunting.
Conventionally, in order to prevent phenomena such as hunting and achieve a preferred AF, methods have been proposed, in which in obtaining a contrast value, a cut off frequency of a filter for extracting a signal within a predetermined frequency band from an image signal is automatically changed depending on a setting of a photographing lens or an AF speed is automatically changed depending on a setting of a photographing lens, so that a unimodal distribution of a contrast value curve does not slope too steeply relative to the AF speed, or the AF speed is not too high relative to the steepness of a unimodal distribution of a contrast value curve.